This invention is related to copending U.S. application Ser. No. 092,024 filed on Nov. 7, 1979 and assigned to the same assignee.
This invention relates to gas flow measuring devices, and more particularly to devices for measuring, for instance, the intake air flow to an engine.
A device, which comprises an electric heater provided on an engine intake pipe and temperature dependent resistors provided upstream and downstream of the heater for detecting the flow rate of the intake air (which is to be measured), has been proposed.
This device is small in size and simple in construction, and it can measure heavy flow. However, since the electric heater and temperature dependent resistors are resistors having the same temperature coefficient, it is impossible to obtain the intake air flow rate that is completely compensated for the influence of the intake air temperature.
FIG. 1 shows an electric circuit which is given for explaining the principles of the gas flow measurement. Referring to FIG. 1, designated at 10 is an electric heater having a resistance R.sub.H (.OMEGA.), at 11 a first temperature dependent resistor with a resistance R.sub.1 (.OMEGA.), at 12 a second temperature dependent resistor with resistance R.sub.2 (.OMEGA.), and at 21 and 22 reference resistors with resistances R.sub.3 and R.sub.4 (.OMEGA.) which form a bridge circuit together with the first and second temperature dependent resistors. The electric heater 10 and first and second temperature dependent resistors 11 and 12 are constituted by resistors having the same temperature coefficient .alpha.. When the intake air which is at a temperature T.sub.a (.degree.C.) is heated by the electric heater so that its temperature is increased by .DELTA.T (.degree.C.), the temperatures of the first and second temperature dependent resistors are respectively (T.sub.a +.DELTA.T) and T.sub.a. At this time, the resistances R.sub.H, R.sub.1 and R.sub.2 are respectively given as EQU R.sub.H =R.sub.OH (1+.alpha.T.sub.a +.alpha.T.sub.H) (1) EQU R.sub.1 =R.sub.01 (1+.alpha.T.sub.a +.alpha..DELTA.T) (2) EQU R.sub.2 =R.sub.02 (1+.alpha.T.sub.a) (3)
(where R.sub.OH, R.sub.01 and R.sub.02 are the resistances values of R.sub.H, R.sub.1 and R.sub.2 at 0.degree. C., and .DELTA.T.sub.H is the temperature rise from the intake air temperature T.sub.a caused by the electric heater.) Denoting the potentials at diagonal points a and b of the bridge respectively by V.sub.1 and V.sub.2, the output voltage .DELTA.V (=V.sub.1 and V.sub.2) of the bridge is expressed as ##EQU1## (where V is the voltage applied to the bridge and electric heater.) Setting R.sub.3 =R.sub.4, R.sub.01 =R.sub.02, the temperature difference .DELTA.T is, from equation (2) to (4), ##EQU2## Meanwhile, ignoring the heat conduction except for that by the air from the electric heater, the intake air flow G (g/sec.), temperature difference .DELTA.T and applied voltage V are related to one another as EQU G.multidot.C.sub.p .multidot..DELTA.T=K.sub.1 .multidot.I.sup.2 .multidot.R.sub.H ( 6)
(where C.sub.p is the specific heat of air under a constant pressure, K.sub.1 is a constant, and I is the current through the electric heater.) By cancelling .DELTA.T in equations (5) and (6) we have ##EQU3## Since in the operation of this device .DELTA.V&lt;&lt;V, and .DELTA.V is controlled to a constant, equation (7) is reduced to ##EQU4## The intake air flow is thus a function of the cube of the current flowing through the electric heater or the cube of the voltage applied to the electric heater. However, since its factor includes a term of the intake air temperature T.sub.a, the measurement of the intake air flow is effected by the intake air temperature. This is undesired for the precise measurement of the intake air measurement.